Resistance element



June l, 1937. I J. R. FISHER 2,082,102

REsIsTANE ELEMEmn Filed April 25, 1955 /N VEA/TOR y J. RF/SHE'R ATTORNEV Patented June l, 1937 RESISTANCE ELMNT Joseph li.. Fisher,Brooklyn. N. Y., assigner to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application April 25, 1935,Serial No. 18,217

6 Claims.

This invention relates to electrical resistance units.

The object of the invention is to produce a resistance unit of silversulphide and a method of 6 making the same.

' A further object of the invention is the produotion of a resistancetunit which will have a high negative temperature coeioient ofresistance; that is, one whose resistance decreases with in- 10 creasein temperature, over the range of temperatures from room temperature toabout 200 C. and whose resistance will be reproducible and reversible,that is, have the same value-at a given temperature regardless of thedirection from l which this temperature is approached.

Another object of this invention is to provide a resistance having ahigh negative temperature coeflicient of resistance whose resistancewill be independent of voltage and frequency. 20 A further object ofthis invention is the provision of `a resistance having a high negativetemperature coemcient of resistance which can be easily formed intounits having a range of resistance depending upon their size and shape.

Many materials are known to have a high negative temperature coeicientof resistance and this property is possessed in common by allsemiconductors to some degree. However, most of these materials areunsatisfactory for use in elec- 30 trical control devices either becausethe change in resistance is not sucient, or because they are not stableand/or reversible, or because of their refractory nature they are notreadily fabricated into a suitable form.

I have discovered that silver sulphide has a combination of propertieswhich make it uniquely suitable for use in electrical devices either asa means of compensating for resistance changes in metallic structures,which have a positive tem- 40 'perature coeicient of resistance, or as acontrol device to operate when a change in resistance reaches a certainvalue.

I have discovered that by the following procedute, resistor umts may beprepared which are 45 suitable for use in electrical devices of theabovementioned nature. Chemically pure silver sulphide in the form of apowder, after sleving through a fine mesh screen, is placed in a steeldie of suitable size and shapeand molded under 50 a pressure ofapproximately ten thousand pounds or more per square inch in a hydraulicpress or ome similar device. By the use of a suitably .dnepowder no heattreatment or binder is necessary, the natural coherence of the particlesbeing 55 sufliclent to produce a mechanically strong unit.

In the event that pure silver sulphide free from excess silver orsulphur is not available, ordinary silver sulphide may be used. However,

if ordinary silver sulphide powder is used ther sister has been formedand ls removed from the mold, it is given aheat treatment, that is, by

heating it rst in sulphur vapor at between 300 and 400 C. forapproximately an hour and then subjecting it to an atmosphere of puredry nitrogen gas at between 200 and 300 C. for' about an hour. Withoutthis treatment the resistor may contain more or less sulphur than isrequired to exactly combine with the silver which will cause a resultingchange in resistance with use.

Referring now to the drawing:

Fig. 1 illustrates, in cross-section, one form. of die which may be usedfor forming applicants resistance element, before force has been appliedto the upper member;

Fig. 2 is a view, similar to Fig. l, except that Vthe upper member hasbeen forced down and has compressed the powdered material located in therecessed portion of the lower member;

Fig'. 3 is the resistance element immediately following its removal fromthe die;

Figui is a view in cross-section illustrating the heat treatingapparatus;

Fig. 5 illustrates one method of applying a conducting surface to theends of the resistance element;

Fig. 6 illustrates the resistance element after it has been sprayed withmolten metal in. accordance with the method shown in Fig. 5; and

Flg. 7 shows the completed resistance element with the terminal wiressecured to the conducting surface of the resistor.

In accordance with the preferred form of the applicants invention. aquantity of chemically pure silver sulphide, in the form of a powder I0is placed in the recess II of the lower die member I2, as shown in Fig.l. When this recess I I has the required amount of powder placedtherein, lthe upper die member I3 is brought downward thus causing theraised portion I4 of the upper member I3 to compress the powder III andenter the recess I I, as shown in Fig. 2. Sullioient pressure is exertedon the upper member I3 to compress the powder I0 into a coherent massI5, as shown in Fig. 3, no binder or other material being necessary.

Applicant has found that pressures somewhere in the neighborhood of tenthousand pounds per square inch have proved very satisfactory, theresulting resistances being very strong mechani-v cally. 0

After the resistanceunit I5 has been removed from the die block I2, itmay in some instances be desirable to heat treat the unit'beforesecuring the terminals thereto. This heat treatment is done bysubjecting the unit I5 to a sulphur vapor which is heated to atemperature of between 300 and 400 C. for approximately one hour andthen heating it in an atmosphere of pure dry nitrogen gas between 200and 300 C. for about an hour.

Applicant may employ any suitable closed vessel which may be heated tothe desired temperature for heat treating his resistances. However, he

has found the apparatus lshown inFig. '4 very' satisfactory.

As shown in- Fig. 4 the resistance unit I5 is suspended in the vesselI6, this vessel containing a s mail quantity of sulphur I1, the sulphur'IIl is caused to vaporize` by' means of the heating element I8 whichraises the temperature in the vessel IS- to a temperature somewhere in'the neighborhood of 300 or 400 C. This temperature *is held atapproximately this level for about an hour, the heat in the interior ofthe vessel being registered on the. thermometer I9, the heatedsulphurvapor in the vessel thoroughly saturating the unit I5. After the .unithas been in this vapor a suilicient length .of time. the 'temperatureislowered to approximately 200 or ,300 -C. and dry nitrogen gas isintroduced through the inlet`20,'

the 'gas passing through the tube 20 down into the vessel' andescapingthrough the outlet 2l,

thus driving oi the sulphur vapor and allowing the resistance unit to besaturated with the corthe center portion covered by a tape or some otherlead-in wires, 21 .and 20 are theny secured tothe tin coated portionsofthe resistance unit by fso similar material 22. The exposed portions23 and 24 of the unit are now sprayedwith a molten metal 25, for'example tin from the gun 26, which vby theway, is the welll-known Shoopprocess.

Thus, a coating of metal is applied to each end of the resistanceunit I5as shown in Fig. 6. The

soldering or any other suitable method.

A resistance unit formed in accordance with the above method is simpleand inexpensive to manu- -facture and the uniformity-of thenishedproduct It will be understood that I do not limit myself to theprecise apparatus herein disclosed as the same may be changed, modifiedor altered without departing from the spirit of the invention and I amonlylimited by the scope of the appended claims. What is claimed is:

1.' A method of producing resistors having a high negative temperaturecoeflicient of restst ance, for use in electrical circuits, vwhichconsists in placing a quantity, of silver sulphide powder between a pairof dies, applying suiiicient pressure 'to said dies to compress thesilver sulphide powder into a coherent mass; removing the resistance'from the lower' die, heating it in an atmosphere oi?l sulphur vapor fora period of approximately. one hour at 300 or 400 C. and-then heating itin a stream of pure drynitrogen-at 200er 300 C. for

approximately one hour.

2. A method of producing resistors having 'ahigh negative temperaturecoeii'icient ofresistance, for use vin electrical circuits, which.consists in placing-a quantityof silver sulphide powder between a pairof dies; applying a pressure of approximately ten thousand pounds persquare inch to said powder by 'means ofl said dies to i'orm the silversulphide powder into a coherent mass, vremoving the resistor' from thelower die, 'heating itA in an atmosphere of sulphur vapor fon a periodof one hour at approximately 300 or 400 C. and

then heating it in a 'stream of pure dry nitrogen. at a temperature ofapproximately 200 or V300" C.

' for about one hour.

3. A method of producing resistors having a high negative temperaturecoeflcient of resist- '.ance, 'for use in electrical circuits', whichconsists in placing a quantity of silver sulphide powderbetween a pairof dies', applying sufficient pressurel to said dies' to compress thesilver sulphide powder into a coherent mass, removing the resistor fromvance, for use in electrical circuits whichgcohsists. in placing aquantity of silversulphidev 'powderbetween a pair of dies, applyingapressure of approximately ten thousand poundspersquare inch to saidpowder by means of said dies to form the silver sulphide powder into a.coherent mass, removing the resistor from the lower die,heating it in anatmosphereof sulphur vapor for a period of approximately one hour at 300or 400 C.,.and then heating it in a stream oipure dry nitrogen at atemperature between 200 and 300 C. for about an hour, masking outaportion oi! said resistance and spraying the exposed portion withmolten metal to'provide contacts thereon.

5. A method .of producing resistors' having a high negative temperaturecoefllcint -of resistance, for` use in' electrical circuits, whichconsists in placing a quantity of silver sulphide powder between a'pairof dies, applying sufficient pressure tosaid dies to compress the silversulphide powderv into a coherent mass, removing the resistor from funit. from the heattreatingvessel, masking out a portion of saidresistance, spraying the exposed portion with molten metal toprovide-contacts thereon and then securing lead wires to said coatedsurfaces.

6. A method of producing resistorsfhaving a high` negative temperaturecoefficient of resistance.- for' use in electrical. circuits, whichconsists in placing a' quantity of silver sulphide powder between apairof'dies, applying'a pressure of ap proximately ten thousand pounds persquare inch in an atmosphere of sulphur vapor for a period 'ofapproximately one hour at1300 or .400 C., and then heating itin a streamof puredry nitrogen at a temperature between 200 and 300 C. for

about an hour, masking out a portion of said resistance spraying the,exposed portion with' molten metal to provide contacts thereon andJOSEPH a.

